Well I was shocked when I looked inside a video magnifier when I had some sight to find that the tubes seemed to be in series, even though they had heaters, they seemed to be powered by some device that simply put a very high voltage across the pair till they struck and then it fell to normal levels. In this case for some odd reason, one could use tubes with a blown heater at one end of each but not the other end. Very peculiar. I think many lighting circuits are designed as a bit of a black art and what happens to tubes is almost impossible to work out. I'd first buy a pair of new tubes and see what happens. If nothing happens then you need a new ballast, or perhaps the fitting as new is going to be almost the same price. Brian
Virtually all multi-tube ballasts run the tubes in series. That doesn't mean they can't provide power for the intermediate heaters.
Starting with high voltage and cold cathodes used to be known as "Instant Start", but that term was subsequently stolen and used to refer to electronic ballasts which were not "instant start" at all, and is hence now meaningless. There are tubes with a single pin contact at each end specifically designed for (the original) Instant Start, but they are not found in the UK (mostly only used in the US, and even that is becoming historic now).
[The original] Instant Start generates considerably more tube wear at each switch-on, so it tended to be used where the tubes were not frequently switched on and off, but was also used where insant-on was an advantage worth having in spite of shorter life. My freezer is in the attached garage, and I wanted instant-on in there, and I found some electronic ballasts which did it (Relco, but if you buy the same part today, it's not instant-on anymore).
Instant Start gear can strike tubes with blown heaters, but if it continues to run the tube, the end will overheat dangerously as the heater only blows when the thermionic emission coating has all gone. Then the remains of the filament change to operate in cold cathode mode which (ironically) generates much more heat, sometimes enough to crack or melt the glass and have the tube fall out or bits of the fitting melt. Good electronic ballasts specifically watch for this and shut down the tube when the thermionic emission coating has all gone (which they detect as the tube turns into a rectifier when one end has lost its thermionic emission coating).
What happens with tubes is completely understood. Older control gear was built as simply as possible, and didn't necessarily cope well with all failure modes. Good modern electronic control gear which is run by a microcontroller can do things like check the filaments at each end before trying to start the lamp, and bail out on a dead tube. It can also spot when someone changes the tube, and restart its startup sequence. Many also automatically work out the tube's electrical characteristics to work out which of several possible tubes they are driving, so manufactures and stockists don't need to handle different control gear for each tube size. When the T5HE range of tubes was being designed, they all deliberately run at the same tube current, so a single constant current ballast can run any one of them in the 14-35W range. It's also the same current as the original 4/6/8/13W T5 tubes, so in theory a ballast could do the whole range from 4W-35W, although I haven't seen one that claims this -- 13-35W is the widest span I've seen. The T5HO range is not so simple - they run at different currents, but many electronic ballasts will automatically work out which of specified list of supported tubes is connected by measuring the electrical characteristics, and then drive them correctly.
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